Why am I pulling in much less than 18K tweets?

When I do an import from a search network, I'm not getting anywhere near the 18K limit, even though there were probably 18K tweets just in 1 day! The search will stop with just 1 or 2K tweets pulled in. Why?
Thanks,
Sal

Twitter decides how many tweets it will give to programs like NodeXL, and it will often be far fewer than what you ask for. Here is what Twitter says about that:

"Please note that Twitter's search service and, by extension, the Search API is not meant to be an exhaustive source of Tweets. Not all Tweets will be indexed or made available via the search interface."